13 research outputs found
RFI Mitigation for the Parkes Galactic All-Sky Survey (GASS)
The GASS is a survey of Galactic atomic hydrogen (HI) emission in the
southern sky observed with the Parkes 64-m Radio Telescope. With a sensitivity
of 60 mK for a channel width of 1 km/s the GASS is the most sensitive and most
accurate survey of the Galactic HI emission in the southern sky. We discuss RFI
mitigation strategies that have been applied during the data reduction. Most of
the RFI could be cleaned by using prior information on the HI distribution as
well as statistical methods based on median filtering. Narrow line RFI spikes
have been flagged during the first steps of the data processing. Most of these
lines were found to be constant over long periods of time, such data were
replaced by interpolating profiles from the Leiden/Argentine/Bonn (LAB) survey.
Remaining RFI was searched for at any position by a statistical comparison of
all observations within a distance of 0.1 deg. The median and mean of the line
emission was calculated. In cases of significant deviations between both it was
checked in addition whether the associated rms fluctuations exceeded the
typical scatter by a factor of 3. Such data were replaced by the median, which
is found to be least biased by RFI and other artifacts. The median estimator
was found to be equivalent to the mean, which was obtained after rejecting
outliers.Comment: accepted for publication in the Proceedings of the RFI mitigation
workshop 29-31 March 2010 Groningen, the Netherland
A confirmed location in the Galactic halo for the high-velocity cloud 'chain A'
The high-velocity clouds of atomic hydrogen, discovered about 35 years ago,
have velocities inconsistent with simple Galactic rotation models that
generally fit the stars and gas in the Milky Way disk. Their origins and role
in Galactic evolution remain poorly understood, largely for lack of information
on their distances. The high-velocity clouds might result from gas blown from
the Milky Way disk into the halo by supernovae, in which case they would enrich
the Galaxy with heavy elements as they fall back onto the disk. Alternatively,
they may consist of metal-poor gas -- remnants of the era of galaxy formation,
accreted by the Galaxy and reducing its metal abundance. Or they might be truly
extragalactic objects in the Local Group of galaxies. Here we report a firm
distance bracket for a large high-velocity cloud, Chain A, which places it in
the Milky Way halo (2.5 to 7 kiloparsecs above the Galactic plane), rather than
at an extragalactic distance, and constrains its gas mass to between 10^5 and 2
times 10^6 solar masses.Comment: 8 pages, including 4 postscript figures. Letter to Nature, 8 July
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The Diversity of High- and Intermediate-Velocity Clouds: Complex C versus IV Arch
We present Far Ultraviolet Spectroscopic Explorer (FUSE) and Space Telescope
Imaging Spectrograph (STIS) observations of interstellar ultraviolet absorption
lines in the Galactic high-velocity cloud Complex C and the Intermediate
Velocity Arch (IV Arch) in direction of the quasar PG 1259+593 (l=120,b=+58
deg). Absorption lines from CII, NI, NII, OI, AlII, SiII, PII, SII, ArI, FeII,
and FeIII are used to study the atomic abundances in these two halo clouds at
V_LSR=-130 km/s (Complex C) and V_LSR=-55 km/s (IV Arch). The OI/HI ratio
provides the best measure of the overall metallicity in the diffuse
interstellar medium, because ionization effects do not alter the ratio, and
oxygen is at most only lightly depleted from the gas into dust grains. For
Complex C, we find an oxygen abundance of 0.093 (+0.125, -0.047) solar,
consistent with the idea that Complex C represents the infall of low
metallicity gas onto the Milky Way. In contrast, the oxygen abundance in the IV
Arch is 0.98 (+1.21,-0.46) solar, which indicates a Galactic origin. We report
the detection of an intermediate- velocity absorption component at +60 km/s
that is not seen in HI 21cm emission. The clouds along the PG 1259+593 sight
line have a variety of properties, proving that multiple processes are
responsible for the creation and circulation of intermediate- and high-velocity
gas in the Milky Way halo.Comment: 12 pages, 3 tables, 3 figures; accepted for publication in Ap
The FUSE Spectrum of PG0804+761: A Study of Atomic and MolecularGas in the Lower Galactic Halo and Beyond
We present an analysis of interstellar and intergalactic absorption lines in
the FUSE spectrum of the low-redshift quasar PG0804+761 (z=0.100) at
intermediate resolution (FWHM=25 km/s) in the direction l=138.3, b=31.0. With a
good signal-to-noise ratio and the presence of several interesting Galactic and
extragalactic absorption components along the sight line, this spectrum
provides a good opportunity to demonstrate the ability of FUSE to do both
interstellar and extragalactic science. Although the spectrum of PG0804+761 is
dominated by strong absorption from local Galactic gas, we concentrate our
study on absorption by molecular hydrogen and neutral neutral and ionized
metals related to an intermediate-velocity cloud in the lower Galactic halo at
-55 km/s, and on absorption from OVI extended to negative velocities. In the
IVC, weak molecular hydrogen absorption is found in 5 lines for rotational
levels 0 and 1, leading to a total H_2 column density of log N = 14.71(+-0.30).
We derive an OI gas-phase abundance for the IVC of 1.03(+0.71-0.42) solar.
Absorption by OVI is found at velocities as negative as -110 km/s, but no
absorption from any species is found at velocities of -180 km/ where absorption
from the nearby high-velocity Complex A would be expected. We suggest that the
extended OVI absorption traces hot gas above the Perseus spiral arm. Finally,
we find intergalactic absorption by an intervening HI Ly betax absorber at
z=0.019 and absorption by HI, CIII and OVI in an associated system at z=0.102.
No intervening OVI absorbers are seen in the spectrum of PG0804+761.Comment: 27 pages, 6 figures; accepted for publication in Ap
Strong Ultraviolet Pulse From a Newborn Type Ia Supernova
Type Ia supernovae are destructive explosions of carbon oxygen white dwarfs.
Although they are used empirically to measure cosmological distances, the
nature of their progenitors remains mysterious, One of the leading progenitor
models, called the single degenerate channel, hypothesizes that a white dwarf
accretes matter from a companion star and the resulting increase in its central
pressure and temperature ignites thermonuclear explosion. Here we report
observations of strong but declining ultraviolet emission from a Type Ia
supernova within four days of its explosion. This emission is consistent with
theoretical expectations of collision between material ejected by the supernova
and a companion star, and therefore provides evidence that some Type Ia
supernovae arise from the single degenerate channel.Comment: Accepted for publication on the 21 May 2015 issue of Natur